Evie Elsaesser was only supposed to survive a few minutes after her birth, but, more than four years later, she’s still here, thanks to an experimental therapy that saved her life.

And she may be small for her age, but she makes her presence known.

“She is really outgoing,” her mother, Lindsey Elsaesser, told ABCNews.com. “She is the daredevil of our kids. She is just keeping us on the edge of our seat.”

Evie was born with hypophosphatasia, or HPP, an inherited bone disease that prevented her skeleton from forming properly in the womb. People with HPP have low levels of an enzyme called alkaline phosphatase, which, among other things, prevents their bones from calcifying properly, making them soft and fragile.

The New England Journal of Medicine

PHOTO: These x-ray images show the bones in a toddler with hypophosphatasia before and after therapy.

Hypophosphatasia can range from mild to severe, said Dr. Michael Whyte, who directs the Center for Metabolic Bone Disease and Molecular Research at Shriners Hospitals for Children in St. Louis. Some adults don’t even know they have it until they get an unusual number of fractures. Severe cases such as Evie’s are diagnosed in the womb – though doctors initially misdiagnosed her with another lethal skeletal disorder.

“You can put your hand onto a newborn’s skull and come away with the impression that you’re feeling a damp paper bag,” said Whyte, who authored the study on Evie’s clinical trial in the New England Journal of Medicine. “There’s no resistance to your lightest pressure on the skull."

About 1 in 200,000 people are born with the severe form of hypophosphatasia, Whyte said.

When Dr. Richard Lutz, a pediatrics professor at the University of Nebraska Medical Center, met Evie shortly after her birth, he was able to diagnose her with hypophosphatasia because she began having unusual seizures, which tipped him off to the enzyme deficiency – and the seriousness of Evie’s case.

“That is a really grave prognosis,” Lutz said, adding that all of the known children with hypophosphatasia and these seizures died within a year of their birth.

But Evie was able to enroll in a clinical trial that would deliver the enzyme to her bones via a regular subcutaneous injection, allowing them to harden and making Evie considerably less fragile.

“When we look at some of her old x-rays, it doesn’t even look like the same child,” Elsaesser said.

Now, Evie goes to preschool three days a week, and even though she’s the same size as her 2-and-a-half-year-old sister, her classmates treat her like any other “normal kid,” her mother said.

“She does know she has limitations,” Elsaesser said.

For instance, Evie’s classmates are learning how to hop on one foot, which she wants to do, too, but she has ankle braces. So she holds the table and hops on one foot. She also likes dancing and swimming, and if her sister plays basketball, she wants to play, too.

“When I get to see her, it’s a real pleasure,” Lutz said. “She’s become physically active. She’s learned how to walk. That was unimaginable when she was first born.”